This invention relates to a catalytic process for the selective synthesis of pseudocumene and durene by methylation of benzene or methyl substituted benzenes using a particular crystalline borosilicate type of molecular sieve. In the resulting product pseudocumene and durene are substantially in excess of equilibrium concentrations among trimethylbenzenes and tetra-methylbenzenes respectively.
There is increasing recognition that petroleum is not a replenishable resource, and that its availability and cost are subject to unpredictable change. It is desirable to seek alternative processes that convert nonpetroleum raw materials to hydrocarbons useful as petrochemicals. The present invention, which utilizes alcohol as raw material, is such a process. Alcohols can be prepared from agricultural products, or from coal, or from petroleum by-products, or from natural gas.
Several polymethylbenzenes are important intermediate products in the chemical industry. Particular di-, tri-, and tetra-methylbenzenes are oxidized to produce polycarboxylic acids and anhydrides of aromatic acids which are useful in the manufacture of synthetic fibers and plastics. Recent interest in manufacturing anhydrides of the most important aromatic tri-, and tetracarboxylic acids, trimellitic (1,2,4-benzene-tricarboxylic acid) and pyromellitic (1,2,4,5-benzene-tetacarboxylic acid), is due to their use, respectively, in trimellitic ester plasticizers and polyimide resins. The commercial method of producing trimellitic and pyromellitic anhydrides is by oxidation of the corresponding tri- and tetramethylbenzenes, pseudocumene (1,2,4-trimethylbenzene) and durene (1,2,4,5-tetramethylbenzene). These aromatic hydrocarbons, which as purified raw materials constitute the largest single items of cost in the manufacture of the anhydrides, are recovered by super fractionation and crystallization. Such processes, as will be realized, have involved high operation costs and have limited yields.
Fractionation of a mixed aromatics stream such as an extracted, heavy catalytic reformate is suitable to recover purified pseudocumene provided non-aromatics are absent. The close fractionation necessary is done in two fractionators (each containing about 100 trays) which remove, successively, aromatics boiling above and below pseudocumene. Typically the fractionation feed of C.sub.9 aromatics from hydroformates contain only about 40 percent pseudocumene. Such low pseudocumene content requires relatively large fractionation towers.
While durene, because of its high freezing point (about 79.degree. C.), could be recovered in pure form by crystallization from a concentrate distilled from C.sub.10 reformate aromatics, the low durene content of such concentrate (about 19 percent) makes this impractical. Durene recovery by a crystallization process is made difficult by the high viscosity of the mother liquor at low temperatures, by the small size of the crystals and by the plate crystal structure which tends to trap liquid and prevent a clean separation of mother liquor from crystals. The crystallization process is easier at higher concentration of durene. Accordingly, methods such as methylating pseudocumene which obtain a durene content in the tetramethylbenzene mixture of about 1/3 may be useful to provide a feed for the crystallization step to recover durene in pure form. The principal problem has been to develop catalysts and reaction conditions to give good yields while avoiding excessive coke formation and catalyst deactivation.
Synthesis of durene by condensation of 1,2,4-trimethylbenzene and methanol over silver-silica-alumina catalyst has been described by Kobayashi et al. in Sekiya Gakkai Shi 13 (10), 775-80 (1970). These workers reported pseudocumene conversion of 33.7 percent and the selectivities for durene and tetramethylbenzenes of 33.8 and 61.8 percent, respectively, under the optimal conditions of 250.degree. C., 2.0 liquid hourly space velocity, 2:1 molar ratio of hydrogen to pseudocumene plus methanol, and 2:1 molar ratio pseudocumene to methanol. The conversion decreased when hydrogen was not used as diluent.
Methylation of pseudocumene using methanol containing less than 1 mole percent (MeO).sub.3 B as a methylating agent in the presence of SiO.sub.2 -Al.sub.2 O.sub.3 -AgO catalyst is described in Japan, Kokai 74 43,930 to Nakano. A mixture of pseudocumene, methanol, and (MeO).sub.3 B was passed at 250.degree. C. and 1 hr..sup.-1 liquid hourly space velocity through a packed catalyst to give a product mixture in which the tetramethylbenzenes were about one-half durene.
Production of durene by toluene methylation using methyl chloride as methylating agent in the presence of aluminum chloride is described by Plyusnin et al. in Tr. Inst. Khim., Ural. Nauchn. Tsentr. Akad. Nauk SSSR 2G, 40-7 (1974). These workers report up to 52.0 percent durene content in the tetramethylbenzenes obtained at 120.degree. C. for 11 hours.
Alkylation of aromatic hydrocarbons utilizing boria containing catalysts has heretofore been described. U.S. Pat. No. 3,217,053 to Kovach et al. claims alkylation of aromatic hydrocarbons with an alcohol or an ether in the presence of a catalyst consisting of a major amount of a calcined alumina base and minor amounts of phosphorus pentoxide and boria, respectively about 0.1 to 20 weight percent and about 0.1 to 10 weight percent. U.S. Pat. No. 3,230,270 to Kovach et al. describes alkylation of aromatic hydrocarbons with methanol in the presence of a catalyst consisting of a calcined alumina base and minor amounts of chromic sesquioxide and boria. The boria is claimed to enhance the utilization of the alkylating agent when said boria is dispersed on the surface of the precalcined gamma-alumina support in direct proportion to the area of the support. Products were reported from C.sub.8 to C.sub.12 without identification of individual isomers.
Various processes for conversion of aromatic compounds are known to employ particular crystalline aluminosilicate zeolite catalysts mixed with a boron-containing additive, such as the disproportionation of toluene described in U.S. Pat. No. 4,029,716. The production of styrene and ethylbenzene by reacting toluene and methanol with a catalyst having an aluminosilicate zeolite of the faujasite structure with potassium, rubidium or cesium cations and containing impregnated boron or phosphorus compounds is described in U.S. Pat. No. 4,140,726.
Although the above background is considered of interest in connection with the subject matter of the present invention, the methylation process described herein uses a catalyst comprising an AMS-1B crystalline borosilicate molecular sieve to achieve unexpectedly high selective production of pseudocumene and durene.
Compared to a conventional thermodynamic equilibrium trimethylbenzene mixture in which the 1,2,4-:1,3,5-: 1,2,3-ratio is approximately 17:6:2, the present invention provides a trimethylbenzene product in which the 1,2,4-trimethylbenzene content may exceed 85 percent. This improved selectivity to pseudocumene reduces the cost of separation of 1,2,4-trimethylbenzene from its isomers which is the most expensive step in the current method employed for the production of pseudocumene.
Further, the methylation process described herein provides a tetramethylbenzene product in which durene content may exceed 95 percent in contrast to a typical thermodynamic equilibrium tetramethylbenzene mixture in which the isodurene:durene:prehnitene ratio is about 5:3:2. The high selectivities to pseudocumene and durene of this invention affords yield and cost advantages to the separation steps to produce these isomers in pure forms.